A typical light integrator includes reflective walls that transmit light in a hollow area of the tunnel via multiple reflections to the output. A homogeneous light distribution is achieved at the output because of the multiple reflections. In particular, the separate walls are assembled together in such a way that a tunnel is formed. The walls can be reflective before being assembled or can be made reflective after the assembly step. Such a light integrator can be used in a light engine for an image projection apparatus. One important parameter of such a light engine is the brightness of the projector. In order to realize bright images strong light sources need to be used. The sources itself as well as the elements in the optical path need to be able to withstand high intensities without being destroyed. Especially the light integrator needs to be able to withstand high amount of light intensity because all of the illumination light needs to transmit through the light integrator and the cross-section of such a light integrator is relative small. Even a small amount of absorption within the light integrator, which normally cannot be avoided, leads to high temperatures. Temperatures equal to or higher than 150° C. can be reached in such applications. Therefore, the light integrator has to be temperature resistant.
A known method for producing such an optical element is to glue the separate planar substrate walls together at the non-optically active surfaces. Typically a UV curing adhesive is used, since this is a fast setting/curing adhesive and therefore allows an easy and inexpensive manufacturing process. In order to perform the assembly, the separate substrate walls are mechanically fixed and the adhesive is applied at the surfaces of the substrates that are not relevant for the optical performance. The mechanical fixation during the gluing guarantees that the relative positions of the substrates are maintained during the complete curing process. After the curing process, when the mechanical strength of the adhesive connection is sufficient in order to stabilize the light integrator, the mechanical fixation is released. If a UV curing adhesive is used, the necessary mechanical strength of the light integrator is reached in a very short time. Typical curing times for commercial available UV curing adhesives are in the range of 5 seconds to 60 seconds. Unfortunately, assemblies based on UV curing adhesives are not temperature resistant for the light intensities as they are more and more used today in image projection systems and therefore they cannot be used for these applications.
In order to realize more temperature resistant assemblies, other types than UV curing adhesives can be used, such as one or two component epoxies, silicone based adhesives, ceramic epoxies, and inorganic adhesives (cements). Of particular interest are ceramic epoxies and inorganic adhesives despite there brittleness. The brittleness of these adhesives gives the impression that they are not suitable for high temperature applications where also severe temperature cycles can occur. However, the coefficient of thermal expansion (CTE) matches closely the CTE of the substrates typically used for light integrators. They provide in general a temperature stable connection between the single substrates. Typical curing schemes for above-mentioned adhesives are exposure to high temperature, humidity or chemical reaction, or initiated by mixing of two components. Unfortunately the curing time of such adhesives is quite long and the time the assembly needs to stay in the mechanical fixation for stabilization increases dramatically. Typical curing times for the above mentioned adhesives are in the range from 1 hour to 24 hours. From a manufacturing point of view such a production method is therefore very expensive and not interesting.
Another approach to realize high temperature resistant optical assemblies is described in WO 01/14923 A1. In this approach a shrink tube is used to fix the single substrates mechanically. Another possibility would be to use one or several adhesive bands wrapped around the assembly, as described in DE 202 17 720. Unfortunately for this method in order to prevent the assembly from collapsing a special shape of the edges of the single substrates is required. Because of this special shape, the integrator cannot collapse and the single substrates are pressed together with a shrinkable tube. It is clear that such a solution has the disadvantage that the mechanical treatment of the edges is difficult and expensive.
Starting from the disadvantages of the methods of the prior art it is the goal of the present invention to provide a possibility to realize temperature stable assemblies not comprising the disadvantages of the prior art such as difficult and/or expensive production processes not suitable for cost efficient mass production.